I am considering adding a few new optional rules to my Runebound game to encourage more PvP.

The one I'm unsure of is this: During PVP combat the defending player may choose to have allies defend for him instead of his hero.

I can't see any drawbacks to doing this but I'm sure the game designers had there reasons for not allowing it. What might they be? Opinions?

The only drawback I can see with allowing that is that a hero with two allies can then save his own defend for the phase he's best at and throw the meat shields up on the other two. Combined with solid armor and a few XP upgrades, this could potentially make the hero nigh invincible, especially if he gets lucky with the allies he draws and/or gets himself set up early so he can terrorize the other players.

I'm not saying it would break the game, but it might not encourage the sort of evenly-balanced, competitive pvp that one would hope for.